Permanent magnet rotors are frequently used in electric machines such as motors and generators. In many applications, permanent magnets are secured to a rotor hub or shaft by any of a variety of means. Care must be taken to assure that such securement prevents the magnets from moving either axially or radially. If axial movement is permitted, one or more magnets may not properly align with an armature with the consequence being that the generator efficiency diminishes. If radial movement occurs, the probability of interfering contact between the rotor and the stator exists, and the resulting frictional drag and/or damage to generator parts comes into existence.
As may be expected, securement of magnets increasingly becomes a problem with increased rotor speeds due to the increasing forces tending to cause magnet movement, particularly in the radial direction, with the increasing centrifugal force accompanying increasing rotational speeds. Thus, in many instances where there are relatively high speeds, and permanent magnet rotors are required, securement of the magnets against both axial and radial displacement is accomplished by sandwiching the magnets between side plates which prevent axial displacement and by winding a fibrous material such as carbon fiber about the peripheries of the side plates and the magnets to provide radial retention. An example of this approach is disclosed in U.S. Pat. No. 4,674,178 issued Jun. 23, 1987 to Navinchandra R. Patel. Patel's patent discloses a rotor assembly having the permanent magnets attached thereto by a fibrous winding which is devoid of loose or potentially loose fibers and ends of the windings. The permanent magnets are disposed in a hub and filler material is located between adjacent ones of the magnets. Axially spaced, opposite side plates are located on the hub to sandwich the magnets and the filler material.
Another method and apparatus for assembling a rotor is disclosed in U.S. Pat. No. 4,918,802 issued Apr. 24, 1990 to Edward J. Schaefer. Schaefer's patent discloses a rotor assembly having a plurality of permanent magnet segments and an outer sleeve or cylinder. A thin-walled metal cylinder is fashioned and arcuate magnet segments are positioned in angularly spaced relation against the interior surface of the cylinder. The segments are displaced radially outwardly to stretch or expand the outer cylinder, and the segments are then supported in the outwardly displaced position. The segments may be displaced by a variety of devices including an expanding mandrel, tapered part of a shaft and a tapered ring.
Another problem encountered with rotors of the type disclosed is in the difficulty in achieving a balance about the intended rotational axis of the rotor. When balance can not be achieved, operation of the generator containing the rotor will result in the generation of vibration which can deleteriously affect the life of bearings and other generator components.
The present invention is directed to overcoming one or more of the above problems.